1. Field of the Invention
The present invention relates to a base drum of an electrophotographic photoconductor and a method for preparing the same, and more particularly, a thin-walled base drum with high dimensional accuracy and a desired surface roughness for use in an electrophotographic photo-conductor, and the method for preparing the above base drum from an electroseamed tube.
2. Discussion of Background
Electrophotographic photoconductors which are now practically used are classified into two types from the viewpoint of shape; one is a cylindrical one and the other is a belt-shaped one. They have their own advantages and drawbacks, but the former is more widely utilized because it is adaptable to a large volume of copies and a rapid copying speed.
As shown in FIG. 1, the above-mentioned cylindrical electrophotographic photoconductor is installed in a copying apparatus in such a fashion that a rotating shaft 3 is fitted in the center of a flange 2a which is secured to one end of a cylindrical base drum 1 of a photoconductor, and a driving shaft 4 is fitted in the center of a flange 2b at the other end of the base drum 1. A photoconductive layer (not shown) is overlaid on the outer surface of the base drum 1.
To prepare this type of base drum 1, conventionally, an aluminum drum (or an aluminum alloy drum) is processed by extrusion and the surface of this aluminum drum is subjected to machining to obtain a mirror surface. This is because the cylindrical base drum of the electrophotographic photoconductor is required to have high dimensional accuracy and uniform surface smoothness.
However, this method of preparing the base drum of this type has the shortcoming that the manufacturing cost is considerably high.
In addition, the material for a photoconductive layer to be coated on the base drum and the thickness of the photoconductive layer are appropriately selected depending on the required photoconductive characteristics. According to the material and the thickness of the photoconductive layer, it is necessary to select the surface profile of the base drum, for example, mirror finish, matte surface or satin surface, and to determine the predetermined surface smoothness (Rmax) thereof, that is, a maximum surface roughness in accordance with JIS B 0601.
For instance, as shown in FIG. 2, the surface roughness of a photoconductive layer 30 varies depending on the thickness of the photoconductive layer even though the surface roughness of the base drum is the same. In the case where the photoconductive layer 30 is thick as shown in FIG. 2(a), the surface roughness of the base drum 1' can be compensated by the thickness of the photoconductive layer 30, and the surface roughness of the photoconductive layer can be improved. In FIG. 2(b), on the other hand, the photoconductive layer 30 is thin, so that the surface roughness of the base drum 1' have a direct influence on the surface roughness of the photoconductive layer 30. Understandably, therefore, the required surface roughness of the base drum varies depending on the kind of photoconductive layer coated thereon.
Consideration is given to the above-mentioned surface profile and surface roughness of the base drum, the base drum of the electrophotographic photoconductor is conventionally prepared by the following methods. A billet is first made out of aluminum or aluminum alloy ingot, and then a tube is formed by hot-extruding (extruded tube), or a tube is formed by drawing the extruded tube at room temperature (drawn tube). Alternatively, an impact-ironing tube is formed by subjecting the billet to cold-impact extrusion and ironing. In addition, a metallic plate or strip is stamped out and subjected to deep-drawing, thereby forming a tube (deep-drawing tube).
The thus obtained tube is further processed into the base drum of the photoconductor, as disclosed below:
(1) A base drum is prepared by machining both end portions and the outer surface of the extruded or drawn tube. Alternatively, the drawn tube is once annealed and again drawn to prepare a base drum. (Japanese Laid-Open Patent Application 64-4753)
(2) A base drum is prepared by curling an end portion of the extruded tube, machining the outer surface thereof and subjecting the tube to ironing. Alternatively, the impact-ironing tube is used as a base drum after subjecting it to machining or without machining. (Japanese Laid-Open Patent Application 59-90877)
(3) A base drum is prepared by machining the deep-drawing tube. (Japanese Laid-Open Patent Application 59-107357)
(4) A base drum is prepared by improving the straightness of an electroseamed tube or worked electroseamed tube by use of correcting rollers, and/or treating the surface of the electroseamed tube by machining finishing, grinding finishing or abrasion finishing, and/or treating the surface thereof by electropolishing or anodizing. (Japanese Laid-Open Patent Application 63-61376)
As previously mentioned, it is necessary to cause the base drum of the photoconductor to smoothly rotate, centering around the driving shaft secured to the flange, as shown in FIG. 1. Therefore, a demand for high accuracy in the coaxiality, the roundness and the straightness of the outer circumference of the base drum, on the basis of the inner circumference thereof, is increasing, and the surface roughness of the outer surface of the base drum, on which the photoconductive layer is formed, is required to be precise.
However, the conventional base drums produced by the above-mentioned methods cannot satisfy such demands for dimensional accuracy. When the base drum is manufactured by extrusion or drawing, the wall-thickness non-uniformity is .+-.10 to .+-.15% on average on the same circumference of the base drum, and the coaxiality on the basis of the inner circumference and the outer circumference of the base drum is unsatisfactory. In addition, the base drum made by the impact-ironing method has the shortcomings that there is non-uniformity in the wall thickness not only on the same circumference of the base drum, but also in the lengthwise direction of the drum and a bent is observed in the lengthwise direction.
In general, the inner surface and the outer surface of the tube for the base drum is subjected to machining to obtain a desired dimensional accuracy and surface roughness. In the case of preparing a thin-walled tube with a large diameter, the distortion of the tube is caused due to chucking in the course of the machining process, and the deformation of the tube is caused after the machining process. Furthermore, the surface of the tube easily shows the waviness and chatters while the tube is machined by a cutting tool. These will lead to the problems of wear-resistance and durability. In particular, it is difficult to obtain a thin-walled base drum having a long length and a large diameter which has high dimensional accuracy and precise surface roughness.
On the other hand, the base drum prepared by deep-drawing does not necessitate the machining process, so that the above-mentioned problems in the extruded or drawn base drum can be solved, and the productivity is high. However, many steps in the sinking process are required to produce a cylindrical base drum with uniform wall-thickness from a metallic sheet. In addition, it is difficult to prepare a base drum with a long length and a small diameter by this method.
When the base drum is prepared by surface-treating the electroseamed tube or worked electroseamed tube by use of correcting rollers, the dimensional accuracy and the surface roughness of the obtained base drum are not satisfactory.
Furthermore, when there is a demand that the base drum have a surface profile except the mirror surface, such as matte surface or satin surface, the above-mentioned electroseamed tubes are subjected to honing, electropolishing or anodizing after correcting process. However, it is impossible to obtain electroseamed tubes which have a desired surface profile uniformly thereon. In addition, to obtain a predetermined surface roughness (Rmax) of the base drum by machining, it is necessary to change the machining conditions, for example, the machining speed and the feed rate, depending on the desired surface roughness, so that the man-hour of a machining process cannot be made constant.